Correlation of exchange coupling and dipolar interactions with magnetization reversal in strontium ferrite/nickel-doped cobalt ferrite composites

This study presents a comprehensive investigation of hard-soft magnetic composites [SrFe 12 O 19 (SFO) and Ni 0.1 Co 0.9 Fe 2 O 4 (NCFO)] synthesized via mechanical ball milling. The composites were analyzed to understand the interplay between exchange coupling, dipolar interactions, and high-field magnetization behavior. X-ray diffraction confirmed the coexistence of hexagonal SFO and cubic spinel NCFO phases without any secondary phase. A steady decrease in crystallite size and a slight change in lattice suggest effective strain accommodation and interfacial interaction. SEM analysis revealed increased particle agglomeration with increasing NCFO content. Magnetic measurements confirmed single-step hysteresis loops, confirming exchange-coupled characteristics. The saturation magnetization decreased with increasing NCFO content, while coercivity was moderately reduced yet remained high due to dominant hard-phase anisotropy. Optimal exchange coupling was observed in the 10 to 20 wt% composites, beyond which partial magnetic decoupling occurred. Law of Approach to Saturation (LAS) analysis revealed increasing defect-related contributions (α 1 ) and high-field susceptibility (χ hf ), indicating enhanced interfacial disorder and spin canting. At the same time, magneto-crystalline anisotropy (α 2 ) remained relatively stable. IRM-DCD and ΔM analyses confirmed a transition from dipolar-dominated interactions to mixed exchange dipolar interactions with increased soft-phase concentration. Thermo-magnetic analysis confirmed distinct Curie temperatures of both phases, validating phase stability. Singular Point Detection (SPD) analysis revealed nearly constant anisotropy fields, suggesting that intrinsic anisotropy depends only on hard-phase. This work lies in the combined application of LAS, IRM-DCD, and SPD techniques to establish a direct correlation between microstructure, exchange coupling, and magnetization reversal behavior. Overall, the study reveals that controlled soft-phase incorporation effectively tailors magnetic interactions and reversal mechanisms, making these composites promising for high-frequency and microwave applications.